Power amplifiers find many uses in the field of electrical and electronic engineering. For example, power amplifiers can be found in audio amplifier circuits, servo-motor controllers, and Radio Frequency (RF) systems, such as are used in relation to wireless digital communications.
In this respect, in many countries, multiple generations of wireless, for example cellular, communications systems presently operate side-by-side. For example, the so-called 4G, or Long Term Evolution (LTE), communications system that is the successor to existing 2G and 3G communications systems provides communications coverage over similar geographic regions. In common, these wireless communications systems comprise a network infrastructure and user equipment, which can for example be portable communications devices. Such communications devices typically receive and transmit signals through the same antenna or antennas. For the transmit signals, RF signals applied to an antenna are typically amplified by a power amplifier prior to application of the signals to the antenna.
However, designing a wireless modem that can achieve efficient power transfer from the power amplifier to the antenna is a challenge shared by communications devices across the generations of systems. In this respect, the RF power amplifier is designed to be connected to a standard load impedance, usually 50 ohms, and optimised to deliver a required power to this load with maximum efficiency. In imprecise terms, the power amplifier is said to be “matched” to 50 ohms. The 50 ohm value is typically selected as it is a universal interface impedance for RF test equipment. However, the term “matched” in this context is a misnomer in the conventional “matched impedance” sense.
For a number of reasons, including the need to support multiple generations of communications systems, most modems for cellular communications devices now have to operate over a wide frequency range. For example, in Europe an embedded LTE communications device may need to operate in licensed frequency bands from 700 MHz through to 2500 MHz, and the antenna has to be reasonably efficient over all the frequency bands within this range. As most applications are size-sensitive, the antenna also has to be quite small, compromising efficiency, especially for the frequency bands below 1 GHz, which are the most useful frequency bands for coverage. For mobile communications devices, particularly smartphones, the commonplace form factor of these devices is of assistance to device radio designers, due to the long dimension of the communications device being, at the lowest operating frequency, approximately a quarter of a wavelength. However, there is no “standard” form-factor and so Original Equipment Manufacturers (OEMs) are at liberty to try to design communications modules into their devices that occupy as little space as possible, often without much consideration for the antenna, which will have to be as small as possible.
In view to the limited consideration given to the antenna, a common approach to improving the performance of an electrically small antenna is by way of an antenna tuning unit. As an antenna is shortened, the impedance of the antenna at resonance becomes reactive, for example capacitive, and this reactance can be cancelled by adding an inductor. An antenna matching network can therefore be provided to modify the impedance presented to a 50 ohm signal source to which the antenna is coupled in order to present a resistive impedance of 50 ohms to the signal source, thereby enabling all available power from the signal source to be transferred to the antenna. All of the power delivered to the antenna will be radiated in the event that the antenna and the matching network are lossless, but of course inefficiencies typically exist.
In relation to the antenna matching network, it is desirable to make such circuits tuneable, so that they can be adjusted to optimise the power transfer from the signal source depending upon the band of operation being used and/or to adapt to other changes, for example antenna detuning caused by the proximity of a user's hand to the antenna. However, in practice available tuneable matching networks, typically switched capacitor networks, are accompanied by undesirable losses, which reduce overall efficiency of a transmitter path. Consequently, antenna tuning is mainly applied in respect of lower operating frequency bands, i.e. longer wavelengths, of a communications device in order to correct the impedance-related inefficiencies attributable to use of an antenna shorter than optimum length.
Furthermore, this approach may involve the use of two “matching” networks: a first network is employed to “match” the power amplifier to 50 ohms to interface to test equipment, and a second network is employed to transform the impedance of the antenna to 50 ohms.
Another known technique for influencing impedance employs so-called “active load-pull”, which is applied for example in a type of power amplifier known as a Doherty amplifier. An example of the Doherty amplifier is described “Controlling Active Load-Pull in a Dual-Input Inverse Load Modulated Doherty Architecture” (Hone et al., IEEE Transactions on Microwave Theory and Techniques, Vol. 60, June 2012, pages 1797 to 1804). The Doherty type amplifier comprises a main power amplifier and an auxiliary (peaking) Class-C power amplifier combined such that at low power levels the auxiliary power amplifier is biased off, but at higher power levels, for example an input signal power from 6 dB below peak upwards, the auxiliary power amplifier starts to take over the supply of a “baseline” load with high efficiency whilst the main power amplifier amplifies in respect of the peak of the input signal.
Another aspect of wireless digital communications that requires consideration with respect to amplifier design is the high Peak-to-Average Power Ratios (PAPRs) sometimes possessed by modulated signals. In this respect, it is difficult to design an RF power amplifier for a communications device that can deliver high efficiency over a wide range of output powers, especially where the communications device is powered by a battery, which by its nature has limited available power, but where the actual transmit power required of the communications device varies over a wide range as dictated by the network infrastructure. Indeed, for spectrally efficient communications systems, for example those operating in accordance with the LTE standard and that employ an Single-Carrier Frequency Division Multiple Access (SC-FDMA) modulation scheme, the PAPR of a signal to be transmitted can be more than 10 dB depending on the bandwidth occupied by the signal and details of the modulation parameters selected from time to time. Where the signal has such a high PAPR, power amplifiers responsible for amplifying the signal to be transmitted conventionally have to operate at a large back-off when stringent linearity requirements must be satisfied. Whilst operating the power amplifier at relatively high output power back-off ensures the LTE signal is not greatly distorted when its envelope is near its peak, the efficiency of the power amplifier reduces with the increase in the amount of output power back-off applied.
One way to compensate for the low operational efficiency of the power amplifier due to the need to operate the power amplifier in the back-off region is to use so-called Envelope Tracking (ET), a technique that allows the supply voltage of the power amplifier to track the magnitude of an envelope of an RF drive signal applied to the power amplifier. In this regard, when the magnitude of the envelope of the RF drive signal is low, the supply voltage is reduced so that the power amplifier operates closer to an optimal efficiency point thereof. As such, a supply voltage to a transistor of a power amplifier is varied so that at every instant the transistor has only just enough voltage headroom for a desired power output, i.e. the transistor operates nearly in compression all the time.
However, envelope tracking places technical demands on the RF power amplifier circuit employed. Envelope tracking requires a modulator comprising a switched mode power supply for high efficiency, having an output voltage that can follow the envelope of the RF drive signal with a bandwidth several times that of the RF drive signal being amplified, for example for about 100 MHz for a 20 MHz LTE carrier signal. Additionally, to implement envelope tracking, the supply voltage of the RF power amplifier circuit has to drop a fixed supply voltage to a variable voltage value in response to an instantaneous envelope amplitude with reference to a tracking table. Furthermore, noise has to be minimised, because any noise, for example switching noise at the output of the modulator, appears directly as amplitude modulation of the carrier signal and increases out-of-band noise. In addition to exhibiting low noise and high efficiency, the modulator has to be provided at a low cost. The design and integration process for an envelope tracking power amplifier is therefore complex.